1. Field of the Invention
The present invention relates to a thermal printer of a type designed to effect a printing with the use of at least one thermal print head for printing on a recording medium and, more particularly, to a platen used in the thermal printer.
2. Description of the Prior Art
With the advent of the age of widespread use of computers, particularly personal computers, the use of thermal printers or dat matrix printers is currently increasing as a computer output device that provides a hard copy of a text. For example, U.S. Pat. No. 4,399,749, issued Aug. 23, 1983, discloses a thermal printer of a type comprising a thermal print head assembly including a row of print heads for printing indicia of different colors by transferring color ink from an ink carrier medium onto a recording medium while the latter is intermittently fed around a cylindrical platen laid parallel to the longitudinal direction of the row of the print heads. The thermal printer disclosed in this publication does not require the use of a movable carriage for the support of the thermal print head assembly as the row of the print heads extends a distance substantially equal to the length of the cylindrical platen. Instead, the plural print heads forming the print head assembly are adapted to be sequentially energized to complete the printing of each line of indicia while the cylindrical platen is intermittently driven to feed the recording medium a distance corresponding to a required line spacing between the neighboring lines of indiecia each time the line of indicia has been printed.
In any event, the above mentioned U.S. patent is merely illustrative of the prior art thermal printer, and the thermal printer utilizing a single print head mounted on the movable carriage for movement together therewith so as to traverse the recording medium in a direction lengthwise of the cylindrical platen is also well known.
In general, the cylindrical platen used in most prior art thermal printers is of such a construction as shown in FIGS. 5 and 6 of the accompanying drawings and is disclosed in, for example, the Japanese Laid-open Patent Publication No. 56-123877 published in 1981. As shown therein, the cylindrical platen generally identified by 1 comprises a support shaft 2 having a tubular elastic layer 3 rigidly mounted on the support shaft 2, said tubular elastic layer 3 being made of, for example, natural or synthetic rubber or any other suitable synthetic elastomer. This cylindrical platen 1 is generally used to support the recording medium 6 turned around that cylindrical platen 1. On one side of the recording medium 6 opposite to the cylindrical platen 1 and between the recording medium 6 and a thermal print head 10 having a plurality of heating elements 10a and mounted on a movable carriage, a length of ink carrier ribbon 7 reeled at one end to a ribbon supply reel and secured at the other end to a ribbon take-up reel extends in a direction generally parallel to the longitudinal sense of the cylindrical platen 1.
As is well known to those skilled in the art, the ink carrier ribbon 7 comprises a strip of heat-resistant base film 8 having one surface confronting the recording medium 6 formed with a layer of thermally fusible ink 9 substantially over the entire length of the strip of base film 8. In practice, the thermal print head 10 is mounted on the movable carriage for movement together therewith and is supported for pivotal movement between printing and inoperative positions in a direction towards and away from the recording medium 6. During the actual printing, the thermal print head 10 is pivoted to and maintained at the printing position until the movable carriage completes its travel from one end of the cylindrical platen 1 to or towards the opposite end thereof to complete the printing of each line of indicia.
The manner in which the printing, that is, the transfer of thermally fusible ink from the ink carrier ribbon 7 onto the recording medium 6 is best shown in FIG. 5. Assuming that the thermal head assembly 10 is pivoted to the printing position at which a portion of the ink carrier ribbon 7 is pressed against the cylindrical platen 1 with the intervention of the recording medium 6 therebetween as shown and an electric character signal descriptive of one alphanumeric character is then applied to the thermal print head 10 , some or all of the heating elements 10a are electrically energized to generate heat. With this heat, a portin 9a of the ink layer 9 on the strip of base film 8 which is then aligned with the heated heating elements 10a is thermally fused and then transferred onto the recording medium 6 after having separated from the strip of base film 8. That portion 9a of the ink layer 9 on the strip of base film 8 which has been trasferred onto and fixed on the recording medium 6 forms a portion of the alphanumeric character represented by the applied character signal.
In order for a particular indicium to be printed clearly on the recording medium 6, it is necessary for the heating elements 10a to be held in contact with the surface of the recording medium 6 under uniform pressure. For this purpose, the elastic layer 3 forming a part of the platen 1 is desirably of a type having a rubber hardness of HS 55 degrees or greater as defined according to the Japanese Industrial Standards.
On the other hand, to accomplish a high speed printing for a given speed of movement of the thermal print heat assembly, it is necessary to facilitate a ready thermal fusion of each portion of the ink layer 9 which is successively aligned with the energized heating elements 10a of the print head assembly 10. This may be accomplished by efficiently transferring Joule heat generated from the energized heating elements 10a to that portion of the ink layer 9 through the strip of base film 8 and, at the same time, minimizing the dissipation of the transferred heat through the platen 1. The dissipation of the heat so transferred from the energized heating elements 10a may be minimized if the elastic layer 3 has a relatively low thermal conductivity.
Hitherto, in order for the platen 1 to have the elastic layer 3 of low thermal conductivity, the elastic layer 3 is generally made of a foamed rubber material or a foamed synthetic resin having a heat retaining capability. The heat retaining capability of the foamed material for the elastic layer 3 permits reduction in thermal conductivity of such material and, therefore, absorption by the elastic layer 3 of the Joule heat generated from the electrically energized heating elements 10a of the thermal print head assembly 10 could be advantageously retarded while permitting the Joule heat to be extensively used to fuse that portion of the ink layer 8 of the ink carrier ribbon 7 for the quick transference onto the recording medium 6.
However, it has been found that the use of the foamed material for the elastic layer 3 of the cylindrical platen 1 poses a problem in that, as shown in FIG. 8 in a cross-sectional representation of the cylindrical platen 1 in relation to the thermal print head assembly 10, the actual use of the cylindrical platen 1 for a substantial period of time results in the permanent formation of indentations 3a left by the heating elements 10a on the outer peripheral surface of the tubular elastic layer 3. The presence of these indentations 3a on the outer peripheral surface of the tubular elastic layer 3 hampers the uniform contact of the heating elements 10a of the thermal print head assembly 10 with the recording medium 6 through the ink carrer ribbon 7 and, therefore, some or all of the indicia to be printed tend to be printed in broken fashion lacking clarity. This in turn results in reduction of the printing quality.
In order to avoid the above discussed problem, it may be suggested that the elastic layer 3 should be made of a hard material having a low thermal conductivity. However, it has been found extremely difficult to manufacture the elastic layer 3 of hard material with the use of a single mold assembly.
By way of example, when the elastic layer 3 is made of the hard material having a low thermal conductivity, the resultant elastic layer 3 would have a porosity decreasing at local areas thereof which would result in the thermal conductivity correspondingly varying at such local areas in the elastic layer 3. Conversely, an attempt to reduce the thermal conductivity of the elastic layer 3 would result in localized increase of the porosity in the elastic layer 3 accompanied by uneven distribution of hardness. Therefore, the suggested use of the hard material for the elastic layer 3 would not alleviate the occurrence of the printing of indicia in broken fashion and, hence, the reduction of the printing quality. Also, the use of the elastic layer 3 made of the hard material could not cope with the requirements necessary to be accomplished for the achievement of the high speed printing.
As an alternative to the cylindrical platen 1 shown in and described with particular reference to FIG. 6, a band-shaped platen 1a made of a metal strip 4 having one surface lined with an elastic layer 5 is also well known as shown in FIG. 7. Even with the band-shaped platen 1a, so far as the elastic layer 5 is made of the same material as that in the cylindrical platen 1, the band-shaped platen 1a remains having problems similar to those discussed in connection with the cylindrical platen 1.